1. Field of the Invention
The present invention is directed to an optical fiber waveguide and more particularly to a single-mode optical fiber waveguide capable of preserving polarization and a method of fabricating the same.
2. Description of Related Art
Communication systems utilizing glass transmission lines for carrying coherent or incoherent carriers in visible and near-visible spectra have become commercially feasible. Such transmission lines have generally utilized fused silica, SiO.sub.2, as the media for glass transmission lines or optical fiber in both the visible and near-visible spectra. An example of such an optical transmission line can be found in U.S. Pat. No. 3,778,132.
The technical literature has also recognized the potential advantages of optical fiber waveguides that can preserve polarization. The potential advantages of a single-mode fiber waveguide able to preserve polarization are well known for use in both instruments and in long distance communications. The technical significance of heterodyne type optical fiber communication has been recognized for a considerable period of time but it has only been with the advent of low loss fibers and semiconductor lasers that commercial interest has been renewed. The ability to use a single-polarization single-mode optical fiber is of particular interest for heterodyne and/or coherent systems.
The ability to preserve polarization through the introduction of anisotropic strain birefringence in an optical fiber was discussed in a theoretical paper by Kaminow, et al., "Single Polarization Optical Fibers: Slab Model," Appl. Phys. Lett., Vol. 34, No. 4, p. 268 (1979). This paper not only recognized the early approach to single polarization fibers through the use of an asymmetrical thick silica outer jacket of a borosilicate preform that was collapsed to form an elliptical cladding region, see Ramaswamy, et al., "Single Polarization Optical Fibers: Exposed Cladding Technique," Appl. Phys. Lett., Vol. 33, No. 9, p. 814, but further dealt with a theoretical analysis to maximize the birefringence. It suggested a slab model with suitable differential doping between the core and cladding to produce a waveguide and a relatively thick jacket to introduce strain as a result of differential thermal contraction as the fiber is drawn and cooled.
Dyott et al. in "Preservation of Polarization in Optical Fibre Waveguides with Elliptical Cores", Electronics Letters, Vol. 15, No. 13, Page 380 (1979), disclosed an elliptical core fiber with polarization preserving properties for long distance communications at relatively long wavelengths.
In the general area of coating optical fibers, a number of different attempts have been made to improve the life of glass fiber optical waveguides through non-metal and metal-jacketing techniques that can provide a hermetic protection of the glass fiber surface. Usually silica fibers are coated with metallic coatings to extend the lifetime of the glass fibers compared to conventionally coated plastic fibers.
U.S. Pat. No. 4,418,984 discloses a multiple coated metallic clad fiber optical waveguide comprising a core portion of high-purity SiO.sub.2 or doped silica of a first index of refraction and a cladding or guiding portion of SiO.sub.2 or any suitable glass material having a slightly lower index of refraction than the core material. A metallic coating of, for example, aluminum, is applied to the glass fiber during a drawing operation immediately after the fiber emerges from the furnace. As the fiber passes through a coating cup, a thin layer of the metal freezes onto the surface of the glass fiber. The dimensions of the opening in the coating cup are such that the surface tension prevents the molten metal from running out. The metallic layer provides good mechanical protection and hermetic sealing against contamination.
Other examples of such teachings in the prior art, can be found U.S. Pat. No. 4,407,561, U.S. Pat. No. 4,089,585, U.S. Pat. No. 4,173,393, Netherlands Patent Terinzagelegging No. 7,602,236 (1976), and the German Offenlegungsschrift 2,826,010 (1979). Further advantages of hermetic jacketing of silica fibers can be found in the Pinnow et al. article, "Reductions in Static Fatigue of Silica Fibers by Hermetic Jacketing," Applied Physics Letter, Vol. 34, No. 1, Page 17 (1979), while a description of applying metal coating to create the hermetic seal about a glass fiber can be found in the Pinnow et al. article, "Hermetically Sealed High Strength Fiber Optical Waveguides," Transaction of the IECE of Japan, Vol. 61, No. 3, Page 171 (1978).
Frequently, the cladding and core materials were prepared as a fused preform of the component glasses prior to the drawing of the fibers. Silica-based glasses have an advantage because they can be worked and reworked without degradation. Silica-based glasses can also be prepared in a very pure form so that the impurity scattering is minimized and they are relatively immune from devitrifying.
U.S. Pat. Nos. 2,772,518, 2,928,716 and 3,788,827 disclose methods of preparing glass optical waveguide fibers with metallic coatings. U.S. Pat. Nos. 2,928,716, 3,347,208 and 3,486,480 disclose various nozzle arrangements for providing an application of a metallic coating to a glass fiber.
The currently preferred approach to fabrication of polarization preserving fibers is the use of a preform geometry which induces high birefringence in the core. In this approach, a composite cladding surrounds a circular fiber core (typically 8 .mu.m diameter). An 80.times.26 .mu.m elliptical jacket of borosilicate glass is formed around the circular core and a silica support tube forms the outer body of the composite. The borosilicate jacket material increases the thermal expansion coefficient of the composite far above that of the support. As a result of the cool-down from approximately 2000.degree. C. where the preform is formed and elliptical shape of the jacket, a large anisotropic stress results over the core region. The direction of high -compression, the short axis of the ellipse, has the higher index of refraction. The anisotropic stress adequately breaks the mode degeneracy which would occur for a simple circular core and clad fiber. This enables the decoupling of the two fundamental orthogonal modes, thereby causing the polarization to be preserved.
One of the disadvantages of this approach is that it relies on the careful preparation of a special preform from which the finished fiber is eventually drawn. This is a costly and exacting procedure.
While the prior art has provided metallic coatings on drawn glass fibers and has further recognized the desirability of anisotropic compressive strains on glass fibers to preserve polarization, there is still a need to provide an improved polarization preserving birefringent fiber optic member and an improved process of manufacturing the same.